A General Technique for Collecting Gas Chromatographic Fractions for Introduction into the Mass Spectrometer SIR: The combination of gas chromatography and mass spectrometry has been recognized as a tool of broad potential for the separation and identification of the components of complex molecular mixtures. General application of this combined method has been hampered, however, by problems encountered in transferring the separated fractions from the gas chromatograph to the mass spectrometer. Direct continuous introduction (3) of the gas chromatographic effluent requires rapid recording of the spectrum and lowers sensitivity because of the large carrier gas dilution of the separated components. Recently developed techniques ( 8 , 11) can concentrate this effluent dramatically, although care appears necessary with the apparatus involved to avoid memory effects, decomposition, and ion source pressure fluctuations. Collection of individual fractions (6, 7 ) for subsequent mass spectrometer esamination is more time-consuming and may risk sample osidation or hydrolysis. However, this avoids loss of sensitivity by dilution and by sample removal in the enrichment process. This also permits separations by gas chromatographs in different laboratories to be monitored by one mass spectrometer. Most fraction collection methods involving condensation of the fractions from the carrier gas face
removed when it contains the sample, as indicated by the detector. Column packing has been shown to be an effective medium for trapping macrosamples from various gas streams for subsequent analysis ( I , 2 , 6, 10, 12). Collectors are constructed from standard melting point capillaries (2 X 100 mm.) packed near one end with 1 to 5 mg. of the GC column packing which has been conditioned under the usual column operating conditions. Use of the same packing as employed in the G C column should ensure a favorab!e partition coefficient. However, packings of lower vapor pressure are frequently used to minimize substrate interferences in the mass spectrum. Collectors may be changed in two seconds, making it possible to trap separate portions of a particular GC peak. The capillary collectors may be conveniently flame sealed for storage or mailing. The recently developed systems for direct solid sample introduction into the mass spectrometer ion source ( 4 ) provide a convenient means for obtaining the spectrum even for submicrogram, low volatility components. The capillary collector is cut to the desired length and inserted through a vacuum lock into the ion chamber of the mass spectrometer. The sample temperature is raised until sufficient sample is being volatilized to record the mass spectrum.
the general problem of providing sufficient cooling for efficient condensation, without forming microcrystallites or droplets (“fogging”) which are swept out of the collector by the gas stream ( 7 ) . This problem is especially severe for the submicrogram amounts of low volatility samples which can now be separated routinely by gas chromatography. The partitioning principle of gas chromatography (GC) can provide an alternate solution t h a t is both simple and general. T h e G C partition coefficient defines the equilibrium ratio of the sample concentration in the liquid substrate and in the carrier gas. When a sample in the gas contacts fresh substrate (column packing) , the sample will be partitioned into the packing, collecting it from the gas. Thus the proposed method can be visualized as a n ordinary GC separation in which the appropriate portion of the column substrate is removed when it contains the desired component. This, then, separates all of that component from the carrier gas, even if present in a trace quantity. I n practice a separate piece of tubing containing GC column packing is attached to the end of the column, effectively extending the column length. The effluent gas stream is split just ahead of this collection tube and a small portion of the stream passed to the detector (Figure 1). The tube is
TO DETECTOR
t
1
EFFLUENT SPLITTER 0 W
COLLECTION GLASS
z a z
50
t
3
POINT
m
a
CAPILLARY
i W
K
STORAGE
ae (M-18).
ANALYSIS
260
280
300
320
340
360
380
400
420
m /e
Figure 2. Mass spectra obtained on direct ion source introduction of GC column packing Figure 1. Steps in collecting a gas chromatographic fraction and introducing it into the mass spectrometer
Top-s% SE-30 on 1.5 mg. Chromosorb P at 95’ C. Bottom-same packing plus 0.1 Mg. pregnanediol diacetate, 9 5 ’ C.
VOL. 37, NO. 10, SEPTEMBER 1 9 6 5
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For more volatile samples cooling is necessary while evacuating the vacuum lock to prevent sample loss. However, for such samples volatilization from the packing can usually be carried out. in an auxiliary high vacuum system connected directly to the ion source, simplifying introduction.
As a representative system a mixture of steroid derivatives was separated on a n Aerograph 600-C Hy-Fi gas chromatograph with a flame ionization detector, hydrogen generator, nitrogen carrier gas, 6 ft. X in. stainless steel column packed with 3% SE-30 on Gas Chrom Q at 250’ C., and a 1 O : l splitter allowing simultaneous collection and detection of the effluent’. Trapped fractions gave spectra of good quality from three different mass spectrometers: Bendix time-of-flight, using a modified Gohlke inlet ( 4 ) ; Hitachi RMU-6A with a Microtek solid introduction system entering between the molecular leak and the ion source; and Consolidated Electrodynamics Corp. 21-110 double-focusing with the 21-086 direct introduction probe. A preliminary check of absolute sensitivity with the latter mass spectrometer was made with collectors containing measured amounts of pregnanediol diacetate. Heating samples of 0.1 pg. gave fairly steady ion currents for 10-20 minutes, so that good qualita-
tive high resolution spectra could be obtained by mass scanning with the electron multiplier detector (Figure 2) , with time for exact mass measurements by peak-matching (9) for several ions. Using photographic plate recording, samples of 0.01 to 10 pg. a t 150’ C. gave total ion currents of 3 X l o + to 1 X coulombs. Thus a G C fraction of about moles can give a high (>10,000) resolution photoplate spectrum which is useful for qualitative analysis. Direct introduction into the ion source of the separated fraction on its chromatographic substrate is also a convenient technique for other separation methods such as liquid-liquid, pa,per, and thin layer chromatography. We are currently investigating such applications, the extension of sensitivity limits, and the automation of this technique. Note added in proof: Since our original report of this work, trapping of G C effluent on column packing for additional G C analysis has been described
(4a). LITERATURE CITED
(1) Beroza., R L ,. J . Gas Chrom. 2, 330 \
,
(1964). (2) Colson, E. R., ANAL.CHEM.35, 1111 (1963). (3) Gohlke, R. S., Ibid., 31, 535 (1959).
S.,Chem. and Ind. 1963, 946. (4a) Kemp, W., Itogne, O., Ibid., 1965,418. (5) Rluller, D. O., ANAL.CHEM.35, 2033 (1963). (6) Novak, J. et al., Ibid., 37, 660 (1965). (7) Ross, W. D., Moon, J. F., Evers, R. L., J . Gas Chrom. 2, 340 (1964). (8) Ryhage, R., ANAL.CHEM.36, 759 (1964). (9) Saunders, R. A., Williams, A. E., “Mass Spectrometry of Organic Ions,” F. W. McLafferty, ed., pp. 354, Academic Press, New York, 1963. (10) Scott, R. P. W., personal communication, April 1965, Unilever Research Laboratory, Sharnbrook, England. (11) Watson, J. T., Biemann, Klaus, ANAL.CHEM.36, 1135 (1964); 37, 844 (1965). (12) widm mark, K., Widmark, G., Acta. Chem. Scand. 16,575 (1962). J. W. AMY E. AI. CHAIT W. E. BAITINGER F. W. RICLAFFERTY
(4) Gohlke, R.
Department of Chemistry Purdue University Lafayette, Ind. RECEIVED for review June 16, 1965. Accepted July 2, 1965. Presented at the Anniversary Meeting, The Chemical Society, Glasgow, April 7, 1965, and at the 13th Annual Meeting on hIass Spectrometry, ASTM E-14, St. Louis, May 1965. Work supported by research grants from the National Institutes of Health (GM 12755) and the National Science Foundation (GP 4335).
Determination of Vinyl Acetate in Ethylene Vinyl Acetate Copolymers Based on High Energy Radiolysis SIR: Recent work in our laboratories on the effect of high energy radiation on several ethylene vinyl acetate copolymers has shown that radiolysis can be an effective tool in the quantitative analysis of their composition, even when the vinyl acetate is present in amounts as little as 1 to 2%. Radiolysis has been used before for studying ethylene polymers but these studies (1-3) have been primarily directed toward analyzing ethylene a-olefin copolymers and identifying the short chain branches in the so-called conventional high pressure polyethylene. Work described here is concerned with a successful extension of this technique in determining vinyl acetate in ethylene vinyl acetate copolymers quantitatively. EXPERIMENTAL
Samples of copolymer in finely divided form and in amounts equal to roughly 1 gram per sample are accurately neighed into glass break-seal tubes of approximately 20-nil. capacity and are sealed off under very high vacuum (at pressures less than 10-5 mm. Hg) after pumping the tubes for a t least 1 hour under this vacuum to rid the 1266
ANALYTICAL CHEMISTRY
samples of any adsorbed oxygen. The samples are then subjected to a measured dose (in the neighborhood of 100 megarads) of gamma radiation from a cobalt-60 or some other suitable source. Following this, the volatile portion of radiolysis product which consists principally of low molecular weight hydrocarbons, carbon monoxide, carbon dioxide, and hydrogen is analyzed by gas chromatography for its carbon monoxide content and its G value determined in the usual manner. G value is the number of molecules produced per gram of sample per 100-e.v. incident radiation dose. RESULTS AND DISCUSSION
When a series of samples of pure polyethylene and ethylene vinyl acetate copolymers of differing vinyl acetate content was subjected to radiolysis under conditions described above with rigorous exclusion of oxygen and their volatile products were analyzed by gas chromatography the copolymers alone gave carbon monoxide as a major component. Further, the amount of carbon monoxide so produced increased s i t h increase in the vinyl acetate content indicating a good possibility of radiolysis affording
a convenient tool for analyzing vinyl acetate content in these copolymers. A calibration curve was constructed by using a number of copolymers of known composition as standards, subjecting the same to known radiation dosage, and calculating their carbon monoxide G values by analyzing the volatile products from each sample for CO content. The CO analysis was carried out using a gas chromatograph. The vinyl acetate content of standard copolymers was determined and crosschecked by three independent methods of analysis, namely, infrared, nuclear magnetic resonance, and saponification. The observed data are given in the upper half of Table I. The calibration curve so constructed from this datai.e., plotting wt. yo vinyl acetate against carbon monoxide G value-is observed to be a straight line that fits the following equation: Wt.
yo vinyl acetate
=
842 X G (carbon monoxide) The method of analysis has been tested using several blends of copolymers and pure polyethylene of known composition, and the ensuing results are
